In an optical communication system, a signal light is inevitably attenuated and is circumvented a waveform distortion when propagating through an optical fiber transmission line due to the transmission loss and chromatic dispersion of the fiber, respectively. For this reason, a predetermined number of repeaters with predetermined intervals are required to construct a long distance optical communication system, so that the optical power and the waveform of signal light are recovered to their initial stages.
One of conventional repeaters for the optical communication system is described on pages 1005 to 1016 of "IEEE Journal of Lightwave Technology, Vol. LT-3, No. 5, October 1985". In the conventional repeater for the optical communication system, the signal light propagated through an optical fiber is received by a photo-detector to be converted to an electric signal which is then amplified and reshaped in its waveform Then, the light source installed in the repeater is modulated by this electric signal, thus it emits the signal light having a reproduced optical power level and a shaped waveform. Then, the regenerated signal light is coupled to an optical fiber for a following stage.
However, the conventional repeater for the optical communication system described above has disadvantages, such that a probability for fault is high therein, a reliability thereof is low, a system size is large, and a system cost is high because a number of electrical devices are used in the repeater for the signal processing.
In order to overcome these disadvantages, an optical amplifying repeater in which the signal light is directly amplified without conversion between optical and electric signals is studied, for instance, as described on pages 51 to 62 of "Journal of Optical Communications, Vol. 4, 1983", and on pages 253 to 255 of "Electronics Letters, Vol. 22, 1986". The optical amplifying repeater is suitably applicable to an optical communication system in which waveform reshaping of the signal light is not under parameters, for instance, that the bit rate is lower than several hundreds M bps and the transmission length is less than 1000 km. Several kinds of semiconductor laser amplifiers are used for such an optical amplifying repeater, such as a Fabry-Perot type or a distributed feedback semiconductor laser which operate with an bias current below its oscillation threshold, and a travelling wave type semiconductor laser having antireflection coatings of a reflectivity less than several percents on both facets. In these optical semiconductor laser amplifiers, the signal light is amplified in the active layer by the stimulated emission effect.
In each of the aforementioned conventional optical semiconductor laser amplifiers, however, there is a problem that the amplification gain fluctuates both with the atmospheric temperature change and the polarization change of the signal light. In more detail, a refractive index of an active layer of a semiconductor laser amplifier is largely varied in accordance with the temperature change thereof. This results in a temperature dependence of the maximum gain wavelength of such a optical semiconductor laser amplifier by approximately 0.1 nm /.degree.C.
There is another problem that an amplification gain of such an optical semiconductor laser amplifier is different for the transverse electric (TE) wave and transverse magnetic (TM) wave. This stems from the fact that mode confinement factor is different between the TE and TM waves because an active layer of the semiconductor laser amplifier is usually asymmetrical and a facet reflectivities are also different between the TE and TM waves.
Despite the polarization dependent gain of the semiconductor laser amplifier described above, it is difficult to propagate the signal light through the optical fiber such that polarization of the signal light is maintained to be constant in the optical fiber communication system. Furthermore, the polarization of the signal light is easily changed when unexpected external disturbance is applied to the optical fiber. For this reason, the optical power level of the signal light after amplification is fluctuated dependent on the polarization of input signal light in a case where one of the conventional optical semiconductor laser amplifiers is applied to the optical amplifying repeater. In order to overcome this problem, it is considered, for instance, that a polarization controller is provided on the input side of the optical semiconductor laser amplifier. However, the polarization controller presently available is not suitable for this application because it has a loss of more than several dB, and the long term stability thereof is still insufficient for such a long term use.
As described above, the conventional optical amplifying repeater is required to be improved in regard to a stability and a reliability, which are presently poor due to the aforementioned amplification gain fluctuation of the optical semiconductor laser amplifiers in accordance with both the atmospheric temperature change and the polarization change of the signal light. Furthermore, there is another problem for the conventional optical amplifying repeater that the transmission line is broken down when the optical semiconductor laser amplifier is damaged during the long term use.